442 Messrs Purvis and Gourtauld, The limitations of the 
only of the reduction of nitrates to ammonia, but also the reduction 
of a portion of the nitrogenous compounds usually called albu- 
minoids. 
So far as we know, no experiments have been published to 
investigate such possible changes ; and the aim of this paper is to 
describe a series of experiments we have conducted to show that a 
copper-zinc couple affects the estimation of nitrates, whenever there 
are complex nitrogenous substances present in solution at the same 
time. 
In estimating nitrates in sewages and sewage effluents by the 
various accepted methods, it is a common experience to obtain 
discordant results ; and our experiments appear to explain the 
causes as regards the copper-zinc couple method. 
So far as the production of ammonia is concerned the investi- 
gation is divided into three parts: (1) the spontaneous decomposition 
of dissolved nitrogenous compounds, (2) the action of a copper-zinc 
couple upon such compounds, and (3) the action of a couple in 
presence of a definite amount of potassium nitrate previously added 
to the solutions. 
Details of the experiments „ 
The preparation of the couples was obviously of paramount 
importance ; and we followed closely the detailed instructions 
given by Dr McGowan and his assistants contained in the Report 
of the Royal Commission on Sewage Disposal, Vol. IV. Part v. p. 17 
et seq. It will be sufficient to refer to that publication for the 
details. The couples were always exactly the same size and shape, 
and were introduced into well-stoppered bottles containing the 
solutions. These solutions varied in strength from 1%’ °/ 0 , 
0'1 %, 001 %, 0-001 °/ 0 °f th e different organic substances. The 
bottles and their 100 c.c. contents were placed in an incubator at 
a constant temperature of 25° C. for 24 hours. They were then 
removed, cooled, and the free ammonia estimated in the usual way 
by comparison with nesslerized standard solutions of ammonia. 
In several of the earlier experiments the incubated solutions were 
distilled in the usual way ; but, as there appeared to be no 
advantage therefrom, the ammonia in the later experiments was 
at once estimated, except in the last sewage samples containing the 
added potassium nitrate, which were distilled. In all the operations, 
water was used containing no free ammonia, and the apparatus 
was always being continually tested for the absence of ammonia. 
Particular attention was given to the absence of organic substances 
in the sodium chloride added to the solutions containing the 
couples, so that there should be no contamination from dirt. 
The sodium chloride added to the couple solutions was always 
ignited for some time in a platinum dish. 
